This study used in vivo assays to detect DC among SCEs. Cat extracts are currently standardized based on Fel d 1 (F). Nine SCEs were tested with Fs of 6.2 to 17.8 U/ml and with F to protein ratios (F/P) of 1.31 to 21.5. SCEs with low F/P were suspected to contain significant non-Fel d 1 (NF) allergen. Two cat sensitive populations were identified by skin testing: F+/NF- subjects; puncture reactive to SCE, but not to cat serum (no detectable F), and F+/NF+ subjects; reactive to both SCE and cat serum. Relative potencies (RP) of the various SCEs were determined by parallel-line skin test (PLST) against a SCE reference which contained significant NF allergen (F=11.8, F/P=1.93). There was a significant correlation (r=0.95, P<0.001) between RP based on PLST and RP based on F in F+/NF- subjects, but not in F+/NF+ subjects (r=0.32, P=0.4). The RP of two SCEs with F content similar to the reference, but with higher F/P ratios (10.6 and 21.5) were only 13-16% of the reference in F+NF- subjects (p=NS). These two SCEs were CD from the reference as were three other SCEs with higher F/Ps than the reference. Three SCEs with F/Ps equal or less than the reference were equipotent to the reference in F+/NF+ and F+/NF- subjects demonstrating they were compositionally similar (CS) to the reference. Because there is a subgroup (about 20%) of cat allergic patients who are sensitive to NF allergens, an assay to detect non-F allergens in addition to F is needed for standardization of cat extracts. This will insure lot to lot consistency in F and NF allergen composition, thus permitting labeling to accurately differentiate CD from CS cat extracts.